1. Field of the invention
The present invention relates to an inspection apparatus utilizing ultrasonic waves, electromagnetic waves or the like, and more particularly, relates to an inspection apparatus such as an ultrasonic non-destructive inspection apparatus utilizing a pulse compression method.
2. Prior Art
Conventional inspection apparatuses of the type explained above are disclosed, for example, in literatures A, B, C as listed below.
Literature A: B. B. Lee and E. S. Furgason, "High-Speed Digital Golay Code Flaw Detection System", in proceedings of IEEE Ultrasonics Symposium, 1981, pp 888 -891. PA0 Literature B: B. B. Lee and E. S. Furgason, "An Evaluation of Ultrasound NDE Correlation Flaw Detection Systems", IEEE Transactions on Sonics and Ultrasonics, Vol. SU-29, No. 6, November, 1982, pp 359-369. PA0 Literature C: B. B. Lee and E. S. Furgason, "High-Speed Digital Golay Code Flaw Detection System", Ultrasonics, July, 1983, pp 153-161. PA0 (1) transmission signal generation means adapted to generate first and second basic unit signals ga(t) and gb(t) in accordance with first and second sequences {a} and {b}, generate a first transmission signal Sap(t) based on the first basic unit signal ga(t) and a third sequence {p}, generate a second transmission signal Saq(t) based on the first basic unit signal ga(t) and a fourth sequence {q}, generate a third transmission signal Sbp(t) based on the second basic unit signal gb(t) and the third sequence {p}, and generate a fourth transmission signal Sbq(b) based on the second basic unit signal gb(t) and the fourth sequence {q}; PA0 (2) transmission means adapted to transmit waves driven by the first, second, third and fourth transmission signals to a target; PA0 (3) reception means adapted to receive first, second, third and fourth echoes reflected from the target to provide echo signals Rap(t), Raq(t), Rbp(t) and Rbq(t) corresponding to the first, second, third and fourth transmission signals; PA0 (4) first correlation means adapted to process by correlation the first and second echo signals Rap(t) and Raq(t) by using a first reference signal Ua(t) generated based on the first sequence {a} and process by correlation the third and fourth echo signals Rbq(t) and Rbq(t) by using a second reference signal Ub(t) generated based on the second sequence {b}; PA0 (5) second correlation means adapted to process by correlation the output of the first correlation means corresponding to the first and third echo signals Rap(t) and Rbp(t) by using a third reference signal Up(t) generated based on the third sequence {p} and process by correlation the output of the first correlation means corresponding to the second and fourth echo signals Raq(t) and Rbq(t) by using a fourth reference signal Uq(t) generated based on the fourth sequence {q}; and PA0 (6) adder means adapted to sum up the respective outputs from the second correlation means.
The construction of a prior art will now be explained by referring to FIG. 1.
FIG. 1 is a block diagram illustrating an inspection apparatus utilizing ultrasonic waves as shown in the above Literature C. The inspection apparatus in FIG. 1 comprises a signal source 1, a digital delay line 2 indirectly connected to the signal source 1, a bipolar converter 3 indirectly connected to the signal source 1 and the digital delay line 2, a transmitter 4 connected to the bipolar converter 3, a bipolar converter 5 indirectly connected also to the signal source 1 and the digital delay line 2, an ultrasonic probe 6, an analog correlator 7 connected to the ultrasonic probe 6, the transmitter 4 and the bipolar converter 5, a display 8 connected to the analog correlator 7, and a system control unit 9.
It is to be noted that the ultrasonic probe 6 is submerged in a water vessel and a target S to be inspected is disposed at the location opposedly facing the ultrasonic probe 6 in the water vessel. It is also to be noted that the analog correlator 7 consists of a multiplier 7aconnected to the ultrasonic probe 6 and the bipolar converter 5 and an integrator 7b connected to the multiplier 7a. Furthermore, logic circuits such as NAND gate and the like are interposed between the signal source 1 and the bipolar converters 3 and 5 as well as between the digital delay line 2 and the bipolar converters 3 and 5. The system control unit is connected to the respective elements as described above to control the system.
Operation of the prior art as shown in FIG. 1 will now be explained by referring to FIGS. 2 and 3.
FIGS. 2 and 3 are waveform diagrams respectively illustrating a transmission signal and a compressed pulse signal provided by the inspection apparatus as disclosed in Literature B.
In FIG. 2, the abscissa is illustrated in the unit of bits and if the unit time is regarded to correspond to the unit of bits, the unit of the abscissa can be taken as the time unit. In Literature B, the unit time corresponding to the unit bit is expressed by .delta.. Therefore, the pulse duration of the transmission signal in FIG. 2 is 63.times..delta..
This transmission signal comprises a signal having a frequency of the base band, and an amplitude of which has been encoded by a special sequence. Encoding of the amplitude will be explained later and the sequence utilized for encoding will be firstly explained.
The utilized sequence is a finite length sequence which has been provided by taking out one cycle of the maximal length sequence (M-sequence) which is a cyclic sequence having a cyclic length of 63 bits.
The M-sequence is described in detail in "Coding Theory" coauthored by Hiroshi Miyagawa, Yoshihiro Iwatare and Hideki Imai, published on Jun. 29, 1979 by Shoukoudo, pp 474-499 (to be referred to a Literature D).
The M-sequence is a cyclic sequence having an infinite length and is a binary sequence components of which are comprised of two elements. The two elements may be allocated with symbols (+) and (-), numeral values +1 and -1 or numeric values 1 and 0 depending on the cases. In the example shown in FIG. 2, a finite length sequence is provided by using one cycle of the M-sequence having the cyclic length of 63 bits and a infinite length.
Encoding of the amplitude of the signal by utilizing this one cycle of M-sequence, or finite length sequence will next be explained.
By providing one element of the finite length sequence with the amplitude +1 and the other element with the amplitude -1, the amplitude for each unit time .delta. is modulated with +1 by the relative value in the order of appearance of these two elements of the sequence. The modulated signal may be called an amplitude-encoded signal.
Similarly to FIG. 2, in FIG. 3, the abscissa is indicated in terms of unit of bits and if a bit as a unit is regarded as the unit of time .delta., the unit of the abscissa may be read as the time.
This compressed pulse signal is an example in which the transmission signal amplitude of which have been encoded by the finite length sequence having a length of 64 bits, is used. This sequence having 64 bits has been provided by adding one bit to the finite length sequence of having 63 bits which was used for generating the transmission signals as shown in FIG. 2. Accordingly, the pulse duration of this transmission signal is 64 .times..delta.. The pulse duration of corresponding echo signals has also the nearly same length.
However, as seen in FIG. 3, a majority of the energy of the compressed pulse signal is concentrated on the central part of the abscissa (time) (a few bit .times..delta.) in the drawing. The portion of the signal located on the central part of the abscissa which have been a considerable amplitude is called as the main lobe of the compressed pulse. The pulse duration of the main lobe is short. This means that the energy of the echo signal which has been substantially uniformly distributed over a long period of time similarly to the pulse duration of the transmission signal has been compressed substantially at one point along the time base. The signal portions having smaller amplitudes at the both sides of the main lobe are called as range side lobes of the compressed pulse.
It is to be noted that the transmission signal as shown in FIG. 2 is generated from the signal source 1 through the digital delay line 2, bipolar converter 3 and the transmitter 4, and the ultrasonic probe 6 is driven by the transmission signal to emit an ultrasonic wave.
The ultrasonic wave emitted into the water in the vessel by the ultrasonic probe 6 will be reflected by the target S and returned again to the ultrasonic probe 6. The echo signal received by the ultrasonic probe 6 will be sent to the multiplier 7a of the analog correlator 7.
The pulse width of the echo signal has nearly the same length as that of the transmission signal. More specifically, the energy of the echo signal has been substantially uniformly distributed over a long duration of time nearly corresponding to the pulse width of the transmission signal (i.e., nearly 63.times..delta. in the case of FIG. 2, and nearly 64.times..delta. in the case of FIG. 3).
The same signal as the transmission signal as described above is sent to the multiplier 7a of the analog correlator 7 via the digital delay line 2 and the bipolar converter 5. The analog correlator 7 is adapted to execute a correlation operation between the echo signal and the transmission signal. This correlation operation will cause the energy of the echo signal, which is substantially uniformly distributed along the time base for a long time duration equivalent to that of the transmission signal, to be compressed substantially at one point along the time base. It is to be noted that the pulse signal obtained through such correlation operation is called the compressed pulse.
The compressed pulse provided by the analog correlator 7 is sent to the display 8 where it is displayed as the final result.
The distance resolution of the conventional inspection apparatus explained above depends on the duration of the main lobe of the compressed pulse (which is referred briefly to as the pulse duration of the compressed pulse). The pulse duration of the compressed pulse is short as described above despite the pulse duration of the transmission signal being long. Accordingly, an equivalent resolution to that of the prior inspection apparatus based on a pulse echo method using a transmission signal with a short pulse duration will be obtained.
On the other hand, the S/N ratio (signal vs noise ratio) becomes higher, as the average transmission energy of the transmission signal becomes larger, and the average transmission energy is larger, as the pulse duration of the transmission signal is larger. Accordingly, according to the conventional inspection apparatus, a higher S/N ratio may be obtained as compared to the pulse echo method using a transmission signal with a short pulse duration.
As explained above, the prior inspection apparatus utilizing the finite length sequence is excellent in resolution and can attain a high S/N ratio.
It is here to be understood that the result of the correlation operation between the echo signal and the transmission signal is represented by a new function with .tau. as a variable, expressed as the following equation: ##EQU1## where r(t) and s(t) respectively represent the echo signal and the transmission signal. This new function is called as correlation function and represents the compressed pulse described above. It is needless to say that the above integration range (-.infin.-.infin.) can be limited to a finite range of time, if either of the echo signal r(t) or the transmission signal s(t) assumes to take value(s) other than zero in the finite time range and to take zero out of the finite time range.
As explained above, according to the conventional inspection apparatus, the correlation operation between the echo signal and the transmission signal is executed by use of the analog correlator 7. However, since the analog correlator 7 consists only of the multiplier 7a and the integrator 7b, operation of varying the variable .tau. in the equation (1) has to be externally executed. In other words, the operation of delaying the transmission signal s(t) by .tau. will be executed by the digital delay line 2 and the system control unit 9 and s(t-.tau.) is input to the multiplier 7a. This means the following.
Since operation of varying the variable .tau. in the relation of equation (1) will not be executed only in the analog correlator 7, this means that the analog correlator 7 is not a correlator in the strict sense of the correlation operation. Furthermore, a single transmission will not provide a wave form of a compressed pulse (correlation function). In other words, what is obtained by a single transmission is only the value of a compressed pulse with regard to a fixed certain value of the variable .tau.. In order to obtain the whole waveform of a compressed pulse, signal transmission must be repeated a number of times by changing the value of the variable .tau. for each transmission. Accordingly, it takes a relative long time until the final result of the whole waveform of the compressed pulse is obtained.
Other correlators for executing strictly (exactly) the correlation operation as expressed by equation (1) will be explained by referring to FIG. 4.
FIG. 4 is a block diagram illustrating another correlator disclosed in Japanese Patent Application No. 1-45316 relating to the present invention.
In FIG. 4, a correlator 10 is constituted by the delay line 10a with output taps, a plurality of multipliers 10b respectively connected to the output taps of the delay line 10a and an adder 10c connected to these multipliers 10b.
The correlator 10 realizes the correlation operation by utilizing the fact that the equation (1) can be transformed as follows: ##EQU2## provided that the transmission signal s(t) is assumed to take zero out of the time range from 0 to T, k and l are integers, .DELTA.t is a sampling interval, K is a constant, t=k.DELTA.t, .tau.=l.DELTA.t and T=k.DELTA.t.
According to the correlator 10, .DELTA.t designates a unit time delay of the delay line 10a between neighboring taps and K designates the aggregate number of taps. When the echo signal r(t) is input to the delay line 10a, an output from the k-th tap (k=1, 2, . . . , K) will be multiplied by the multiplier 10b with a weight value s(k.DELTA.t) which has been prepared in advance. Subsequently, the adder 10c is caused to add outputs from all the multipliers 10b, whereby the result of the addition is equivalent to equation (2).
According to this correlator 10, operation of changing the variable .tau. corresponds to inputting the echo signal r(t) to the delay line 10a in the sequential timing. The echo signal r(t) is naturally input from the ultrasonic probe 6 in the sequential timing. Accordingly, the operation of changing the variable .tau. is automatically executed. Namely, according to the correlator 10 shown in FIG. 4, the time waveform of the compressed pulse can be obtained by only one transmission and in real time.
However, when the duration of the transmission signal, namely T becomes larger, the delay line 10a having a greater many number of taps will be required, and hence a greater many number of multipliers 10b will also be required. Further, also the adder 10c having a greater many number of input terminals will be required when a greater many number of the multipliers 10b is required. As the number of multipliers 10b and the number of input terminals of the adder 10c increase, the operation speed of the correlator 10 decreases. Moreover, the cost of such a correlator will be more expensive.
Furthermore, as seen in FIG. 3, the conventional apparatus has such a drawback as the level of the side lobes of the compressed pulse being relatively high.
Accordingly, the prior inspection apparatus as explained above takes a great deal of time to obtain the compressed pulse as a final result, while the operation speed has to be made slower if an attempt is to be made to realize real-time inspection by shortening the time required for obtaining the compressed pulse and the cost of the apparatus becomes expensive.
There is also a problem that the level of the side lobes of the compressed pulse is high.